# Mythbusters: Blow your own sail

#### Trev

Hi, I know this has been done but a quick read through on here does not seem to address the household discussion we had on this topic. The mythbudgets claimed to have defeated Newtons laws by putting a sail on one end of a fan boat and turning the fan around. I predicted it would not work, but by the end of the show the boat appears to be pulled along by the sail, albeit slowly.
Reading the existing thread I could not see the explanation I landed upon as follows, perhaps you can tell me why I am wrong.
The boat is being propelled by atmospheric pressure. The moving body of air between the fan and the sail has a lower pressure relative to the still air around the boat at right angles to the flow of air to the sail. As a consequence the air around the boat rushes into the space and results in increased mass pushing on the sail.

Related Classical Physics News on Phys.org

#### A.T.

The mythbudgets claimed to have defeated Newtons laws
Did they really claim that? The behavior is completely consistent with Newtons laws, and similar to thrust reverses on airliners.

The moving body of air between the fan and the sail has a lower pressure relative to the still air around the boat at right angles to the flow of air to the sail. As a consequence the air around the boat rushes into the space and results in increased mass pushing on the sail.
Nobody had any doubts how the force on the sail is created. The issue was whether it can be greater than the recoil of the fan in the opposite direction.

#### Trev

From memory they did say "appears to have violated Newtons Laws".

Re your second point my apologies, re reading the post over a little more time, that the boat moves forwards because air is accelerated from an already moving speed by the propeller, and then comes to a dead stop, or indeed a negative speed at the sail, creating a greater force against the sail than the reaction force against the propeller.

The conversation goes on to discuss at some length 'suck' which is of course another way of say atmospheric pressure, I think my point would be to make that the same pressure is pushing in from the sides of the column of air between the propeller and the sail, and this is contributing, as the system otherwise would be lossy to the extent that the force on the sail could not exceed the force on the propeller, but I am late to the conversation.

Still, nice forum, look forward to poking around in here, I'll read with greater care before posting next time :-).

#### DrStupid

From memory they did say "appears to have violated Newtons Laws".
Maybe in the sense of "it looks like it violated Newtons Laws". If I recall correctly they explained the effect as thrust reversal.

#### DaveC426913

Gold Member
I suppose the original conclusion that the fan+sail can't work is based on the assumption that air has no inertia. If it starts at the fan and hits the sail and simply stops, then the forces cancel out.

But imagine replacing the fan with a ballbearing gun, and the sail with a plate of steel. The ballbearings will rebound off the plate and escape to the rear of the vehicle, producing thrust. We don't normally consider air to have inertia, but if you get enough of it moving, it certainly will.

So, yes, like thrust reversal in a jet engine, but ... reversed. So, thrust reversal reversal.

#### A.T.

I suppose the original conclusion that the fan+sail can't work is based on the assumption that air has no inertia.
If that had been the assumption, then the conclusion would have been that no wind-powered vehicle can work. The wrong assumption is more likely that the collision of the air mass with the sail just stops the air, like in a inelastic collision. While in fact it can reverse it's momentum.

#### Andrew Mason

Homework Helper
From memory they did say "appears to have violated Newtons Laws".

Re your second point my apologies, re reading the post over a little more time, that the boat moves forwards because air is accelerated from an already moving speed by the propeller, and then comes to a dead stop, or indeed a negative speed at the sail, creating a greater force against the sail than the reaction force against the propeller.
If the air is being sucked in from the back and pushed toward the sail then it is not possible for the boat to move forward due to the air pushing on the sail. No matter how you configure it, the force on the air caused by the propeller is going to be equal and opposite to the force on the propeller(/boat) caused by the accelerating air.

If the air is being sucked in from the front and pushed toward the sail and allowed to flow out the back then the forward motion is due to the force of the air on the fan, not the sail. This, of course, is how an airboat works - like a propeller airplane.

If the air is being sucked in from the side and the sail is used to deflect the air toward the rear, then the boat could move forward. This is just a less efficient method of moving air toward the rear. The air that is deflected backward pushes forward on the sail and, therefore, the mast pushing the boat forward.

There may also be another more complicated reason that has to do with the shape of the sail as an airfoil if there is external movement of air. A sailboat can sail into the wind, for example. It is not pushed by the wind. Rather the sail shape creates a low pressure region on the curved side of the sail and the boat is drawn forward by the difference in pressure.

AM

#### rcgldr

Homework Helper
As long as the sail is large enough and shaped so that the reversed flow doesn't significantly interfere with the flow being sucked in by the fan it will work. Assuming still air and the ground as a frame of reference, the change in momentum of the air sucked in by the fan is m v, where v is the speed of the air sucked in as it goes from zero to the speed it exits the fan at. The change of the momentum of the air off the sail just has to be reversed from forward to backwards with some non-zero backwards velocity so that the change in momentum of the air off the sail is greater than m v, perhaps 1.25 m v would be enough.

As the blow your own sail boat moves forward, then it's sail also moves a horizontal column of air forwards with it, part of which eventually ends up being sucked in by the fan, so that the air being sucked in by the fan has an initial non-zero forwards velocity.

As an analogy, imagine there are a bunch of balls floating in the water, and that the boat has a trampoline like surface for a sail. You grab a ball from the water and throw it at the trampoline where the ball ends up moving backwards relative to the water after it bounces off the trampoline sail. As the boat moves forwards you grab another ball from the water and repeat the process.

A bit off topic, a sail boat can sail into the wind because it interacts with both the air and the water. A sail boat on a frictionless surface could not sail upwind. Ignoring the drag component, the water and sail have a Newton third law pair of force component perpendicular to the direction of travel of the boat (wrt the water), which allows the sail to divert the apparent wind to produce thrust (wrt to direction of travel of the boat). The coefficient of lift versus the total drag on the sail boat determines how small the angle to directly upwind the sail boat can sail upwind. In the case of an ice boat, the drag is much less and an ice boat can tach with an upwind component of speed greater than the speed of the wind. So can an DUWFTTW (directly up wind faster than the wind) cart that uses a prop driven by it's wheels.

Full scale demo of blow your own sail by MythBusters:

Last edited:

#### A.T.

If the air is being sucked in from the back and pushed toward the sail then it is not possible for the boat to move forward due to the air pushing on the sail.
Of course it's possible. See MB video above and a more controlled experiment below.

No matter how you configure it, the force on the air caused by the propeller is going to be equal and opposite to the force on the propeller(/boat) caused by the accelerating air.
Wrong, see post #6.

#### Andrew Mason

Homework Helper
Andrew Mason said:
No matter how you configure it, the force on the air caused by the propeller is going to be equal and opposite to the force on the propeller(/boat) caused by the accelerating air.
Wrong, see post #6.
Of course it's possible. See MB video above and a more controlled experiment below.
You are saying that Newton's Third Law does not apply? Read my post carefully.

AM

#### A.T.

You are saying that Newton's Third Law does not apply?
No I'm saying this part is wrong:
If the air is being sucked in from the back and pushed toward the sail then it is not possible for the boat to move forward due to the air pushing on the sail.

#### Andrew Mason

Homework Helper
That is a slightly different issue. I was responding to your #9 post.

AM

#### thankz

wouldn't this be considered a standing wave?

#### Andrew Mason

Homework Helper
No I'm saying this part is wrong:
Andrew Mason said:
If the air is being sucked in from the back and pushed toward the sail then it is not possible for the boat to move forward due to the air pushing on the sail.
For the boat to move forward there has to be a net movement of air rearward. But that can't happen if you are moving air into the fan from the rear at the same rate that you are pushing it backward (and that assumes a perfect elastic rearward reflection from the sail, which is never going to happen). A reverse thruster on a jet engine works because the fuel gases combined with the air are pushed toward the direction of travel faster than the incoming air. And that only works for a brief time because the air supply rapidly decreases.

AM

#### A.T.

For the boat to move forward there has to be a net movement of air rearward.
Yes

But that can't happen if you are moving air into the fan from the rear at the same rate that you are pushing it backward
Yes, so you have to push it backwards more, which the sail does.

(and that assumes a perfect elastic rearward reflection from the sail, which is never going to happen).
No, it happens as soon as the reflection from the sail is not perfectly inelastic, but still far from perfectly elastic.

A reverse thruster on a jet engine works because the fuel gases combined with the air are pushed toward the direction of travel faster than the incoming air. And that only works for a brief time because the air supply rapidly decreases.
A jet engine accelerates air, just like a fan does. There is no reason why a thrust reverser should work on one, but not on the other.

And that only works for a brief time because the air supply rapidly decreases.
A thrust reverser can work continuously, not just for a brief time.

Last edited:

#### Andrew Mason

Homework Helper
Yes, so you have to push it backwards more, which the sail does.
How? If I am over-simplifying something, please correct me. But you will have to explain it.
No, it happens as soon as the reflection from the sail is not perfectly inelastic, but still far from perfectly elastic.

A jet engine accelerates air, just like a fan does. There is no reason why a thrust reverser should work on one, but not on the other.
Here's my reasoning: A jet engine crams air and fuel into a chamber and ignites it. It expels more gas rearward than was brought in and the stream is much narrower and faster than the incoming stream. So if you were to direct that stream forward, the reverse thrust would be greater than the forward propulsion caused by the fan blades sucking air in from the front. A fan or propeller just pushes air. It does not concentrate it. The rate at which mass flows into the propeller has to equal the rate at which mass is expelled behind the propeller. Since the area behind the propeller is the same as the area in front, there is no concentration of the air so it can't expel the air at a greater speed than it takes it in. So putting a thrust reverser behind a propeller and turning up the forward propeller speed would not work. To create reverse propulsion with a propeller you have to reverse the pitch so that it is pushing air forward.

A thrust reverser can work continuously, not just for a brief time.
Where does the intake air come from if the plane is not moving forward or moving forward quickly enough to supply the intake fans?

AM

#### A.T.

The fan accelerates air from 0 to v. The sail reverses the direction from v to something < 0. Which of the two constitutes more momentum transfer, and will thus exert more force on the boat?

#### rcgldr

Homework Helper
The air flow through a fan does get concentrated, except that due to viscosity, the flow interacts with the surrounding air. The air accelerates as it approaches the plane of the fan (or propeller), then it's speed remains about the same as it crosses the plane of the fan (or propeller), where there's a pressure jump (the pressure jump is how the fan (or propeller) perform work on the air). The now higher than ambient air continues to accelerate as it's pressure returns back to ambient. Ignoring viscosity affecting the surrounding air, the mass flow rate remains constant, so the cross sectional area of the flow decreases as the flow speed increases. There is a NASA article about this:

http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

A.T.'s post pretty much explains why the sail does not have to be 100% elastic.

About the reverse thrusters on a jet, at some airports, the terminals are blast tolerant, and commercial jets can use reverse thrusters to back away from the terminal.

Last edited:

#### CWatters

Homework Helper
Gold Member
Force = mass * acceleration so lets look at the acceleration of the air...

The air going through the fan is accelerated from 0 to X m/s.
At the sail the air is reflected or "accelerated" from X to -X m/s.

So the rearward force on the fan is proportional to X and the forward force on the sail is proportional to 2X.

So there is a net forward force proportional to X.

Edit: Clearly the sail may not reflect all the air but it doesn't have to. If any is reflected there will be a net forward force.

#### Andrew Mason

Homework Helper
A.T.'s post pretty much explains why the sail does not have to be 100% elastic.
I never said that it had to be 100% elastic. I said that there has to be net airflow rearward. You can do that any way you want. You could do it with a jet engine with the sail or something like a metal plate directing the expelled gases rearward. I just have difficulty understanding how you could get a net rearward stream of air if you are sucking it in from the rear at the same rate as you are pushing it toward the sail. If the flow of air into the fan is vdm/dt and the rearward flow deflecting from the sail is -vdm/dt (which is the best you can get), how do you get a net rearward flow of air? In actual practice, the rearward flow will be much less than v due to dispersion of the air.

AM

#### A.T.

the rearward flow deflecting from the sail is -vdm/dt (which is the best you can get)
Why should that be the best you can get?

#### A.T.

If you have -vdm/dt of force on the sail
You don't. If the sail reverses the flow from v to < 0, then the force on the sail is more than vdm/dt.

#### DaveC426913

Gold Member
A jet engine crams air and fuel into a chamber and ignites it.

A fan or propeller just pushes air.
Both of them add energy to the air. One from fuel the other from electricity.

They do work on the air and that is translated to work on the craft.

Your assertion that a "fan just pushes air" seems to assume it doesn't add usable energy to the system. A jet engine simply adds somewhat more.

Last edited:

#### rcgldr

Homework Helper
how do you get a net rearward flow of air? In actual practice, the rearward flow will be much less than v due to dispersion of the air.
The videos are proof that the blow your on sail models can move forwards, although the speed is slow since the sail also acts as a air dam pushing the air in front of it forwards. Momentum is conserved, and if the sail boat moves forward, then there has to be a net opposing change in momentum of the air. Since the back flow velocity off the sail is less than the forwards flow out of the fan, the back flow must involve a larger volume (mass) of air, so the flow that originated from the fan and deflected by the sail apparently interacts with the surrounding air.

The fan is also operating in a near static situation, so I'm not sure how much of the intake flow starts off with a component perpendicular to the flow at the fan. I'd be curious to see a smoke test done on the intake side of a fan.

Last edited:

#### Andrew Mason

Homework Helper
The videos are proof that the blow your on sail models can move forwards, although the speed is slow since the sail also acts as a air dam pushing the air in front of it forwards. Momentum is conserved, and if the sail boat moves forward, then there has to be a net opposing change in momentum of the air. Since the back flow velocity off the sail is less than the forwards flow out of the fan, the back flow must involve a larger volume (mass) of air, so the flow that originated from the fan and deflected by the sail apparently interacts with the surrounding air.
I think that is the key. It has to impart backward momentum to a larger mass of air than is flowing out of the fan.

You will notice in the film that it only provided forward net force if they kept changing the fan's direction. If the fan is kept pointing directly toward the sail the pressure builds up behind the sail and can escape only sideways (and unless the sail is perfectly positioned, more escapes from one side than the other and the boat starts going in circles). However, if the fan is then directed off the sail, the air under pressure between the fan and sail can escape backward and this rearward momentum of the air produces forward momentum of the boat. Unless that pressure is allowed to build up and then released by changing the fan direction, there would be no forward momentum to the boat.

You can get a watermelon to move toward the shooter by putting a hard metal sheet in front of it and hitting it with soft hunting bullets. The bullets flatten on the initial impact and in passing through the watermelon built up pressure in front of the bullet inside the watermelon. When the bullet reaches the other end of the watermelon, the contents explode out of the melon in a forward direction sending the melon backward with more momentum than the bullet carried into it. Unless that pressure is built up and then released to a much larger mass of watermelon than the bullet mass, there is no way the watermelon will move backward.

AM

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving